This disclosure pertains generally to sensor characterization and in particular pertains to a method and system for vicarious spatial characterization of a remote image sensor. By vicarious characterization of a remote image sensor it is meant characterization of the image sensor after deployment of the image sensor to its operational environment (e.g., on an aircraft or spacecraft).
Airborne, and space-borne remote sensing systems for geospatial survey, observation, target recognition, and other applications are increasing in use. For example, the remote sensing systems may be employed to detect anthropogenic and natural effects in climate change, geologic morphology and chemistry, hydrology, vegetation health, as well as to identify and distinguish between friend and foe military targets (target recognition), drug operations, terrorist activities, damage assessment, and emergency routes for responding to natural disasters.
In order for the remote sensing system to operate as intended to quantify physical properties of an observed object, image data obtained by the remote sensing system and the physical properties of the observed object are quantitatively linked. Thus, providers of remote sensing systems strive to provide adequate methods for addressing stability and accuracy requirements imposed by the user community to define and validate sensor spectral, spatial, and radiometric performance, and in turn establish the level of confidence for data exploitation.
Development of panchromatic and multispectral sensing systems continue to move toward increasing spatial resolution in response to the fact that most targets of interest are contained in only a few pixels or even sub-pixel (i.e., an image area of the target is less than a pixel area). Generally, each image is composed of a plurality of pixels, with the radiation sensed by each pixel analyzed to determine the physical content and make up of the target contained in a pixel area. However, for small targets, blur spots due to optical diffraction, electronic readout, sensor motion, atmospheric scattering, or any combination thereof, as well as other potential natural phenomena or technical issues, can smear light into nearby pixels spatially disconnected from the target and thus blur the image of the object. Multispectral and hyperspectral sensors collect image data across a plurality (e.g., tens to hundreds) of spectral bands in which the blurring or smearing effect can vary with wavelength.
As a result, knowledge of the spatial performance (i.e., sensor point spread function) is applied as part of a calibration process so as to achieve effective small targets. Hence, one element in the operation of airborne and space-borne imaging systems is sensor calibration on the ground before launch or flight. However, physical conditions within the imaging system or conditions in the atmosphere between the imaging system and the desired target or object may change from the calibration laboratory setting in such a way so as to skew the calibration values. Therefore, the sensor characterization on the ground in the laboratory becomes suspect until validated after deployment of the sensor. The validation of the characterization after sensor deployment or vicarious characterization of the sensor provides an absolute information of the sensor characteristics to ensure validity of the laboratory or ground based characterization or to correct the laboratory characterization to take into account conditions that may have occurred after deployment of the sensor.
However, vicarious spatial characterization of an image sensor can be challenging. Prior methods for performing vicarious characterization of a remote image sensor use either large extended area bar targets, edge targets with varying levels of differential reflectance, or images of naturally occurring point sources such as stars. However, these methods have issues with either contrast (e.g., over large spectral ranges) and thermal management issues (particularly when Earth observing sensors slew to deep space).
Hence, there is a need in the art for a system and method for vicarious spatial characterization, i.e., characterization after deployment to a remote site, of a remote image sensor.